1. Field of the Invention
The present invention relates to communications systems, and more particularly, to symbol timing synchronization in an OFDM system.
2. Description of the Prior Art
In orthogonal frequency division multiplexing (OFDM) communication systems, data is modulated by tones used by the system. A transmitter transmits OFDM symbols contiguously in a packet and therefore conveys a continuous information data flow to the receiver. The transmission, however, will be distorted by the unknown characteristics of the channel. Therefore, the received OFDM symbols in a packet may not be recognizable as the original transmitted OFDM symbols. In situations like this, the receiver needs to perform certain signal processing tasks before it can retrieve the original OFDM symbols. The most significant adverse effect in a channel is Inter-Symbol Interference (ISI). When ISI occurs, waveforms of contiguous OFDM symbols interfere with each other leading to unrecoverable distortion. To mitigate the effect of ISI, some OFDM based systems, such as VDSL or HomePlug, introduce a so-called circular prefix and/or suffix. These prefixes and/or suffixes are inserted between contiguous OFDM symbols and act as guardian intervals to reduce the possibility that waveforms interfere with each other.
At the receiver side, therefore, it is crucial to reduce distortion caused by ISI as much as possible such that contiguous OFDM symbols can be separated, i.e., the boundaries of each OFDM symbols can be set such that no substantial “leakage” from neighboring OFDM symbols exists. The task of properly determining the start and end of each individual OFDM symbol and then compensating any timing offset is called symbol timing synchronization, or symbol synchronization. Often a training signal sequence is used to facilitate this task.
Symbol timing synchronization refers to the task of finding the precise moment when an individual OFDM symbol starts and ends, and is an essential task for a digital communication system. After a packet detector has provided an estimate of the start of the packet, a symbol timing synchronization algorithm refines the estimate to sample level precision. Most of the existing methods of symbol timing synchronization fall into two major categories. The first is based on signal processing in frequency domain. This type of method first calculates the fast Fourier transform (FFT) of the received signal and the phase angle of each tone. The differences between the phase angles of each tone and a set of pre-determined reference phase angles are calculated. Then the phase differences are processed by, for example, fitting to a linear regression model. In this case, the timing offset can be estimated from the slope of the regression line. The second category is based on signal processing in time domain. Usually the received signal is correlated with a pre-determined reference time-domain signal. Then, for instance, the peak of the correlation function obtained can be used to determine the timing offset.
Time domain methods often suffer significant performance degradation when strong narrowband interference (NBI) or Gaussian noise is present. Properly designed frequency domain methods can achieve excellent performance. However, they often suffer from heavy computation required by processing the phase angles obtained from the FFT of received signals. Also, due to their reliance on sophisticated signal processing algorithms, often they are not robust enough under conditions of severe noise. In this invention, a frequency domain method with low complexity for OFDM symbol timing offset estimation is described.